KR20050008832A - Steel for crude oil tank and method for manufacture thereof, crude oil tank and method for protecting corrosion thereof - Google Patents

Abstract

Industrial Applicability The present invention is for welding structures that exhibit excellent overall corrosion resistance and local corrosion resistance against crude oil corrosion generated in a forced oil tank, and can suppress formation of corrosion products (sludge) containing solid S. Crude Oil Tanker Steel, Crude Oil Tanker Steel Manufacturing Method, Crude Oil Tanker and Crude Oil Tanker Method are provided, and C: 0.001 to 0.2%, Si: 0.01 to 1% by mass as the basic components. 2.5%, Mn: 0.1-2%, P: 0.03% or less, S: 0.007% or less, Cu: 0.01-1.5%, Al: 0.001-0.3%, N: 0.001-0.01%, Mo: 0.01- 0.2%, W: 0.01% to 0.5%, or more, and more preferably, satisfies the solid solution Mo + employment W ≥ 0.005%, showing full corrosion resistance and local corrosion resistance in the crude oil tank environment, It also suppresses the formation of corrosion products (sludge) containing solid S.

Description

STEEL FOR CRUDE OIL TANK AND METHOD FOR MANUFACTURE THEREOF, CRUDE OIL TANK AND METHOD FOR PROTECTING CORROSION THEREOF}

Welded structural steels with excellent strength and weldability are used in forced oil tanks for transporting and storing crude oil such as crude oil tankers and above-ground and underground crude oil tanks. The problems of corrosion damage in crude oil tanks that have been required to be solved are 1) reduction of corrosion of steel plate, in particular, reduction of local corrosion damage in the form of a relatively rapid progression, and 2) gas phase part that causes sludge ( It was reduction of the solid sulfur which precipitates to the steel plate surface in the phase section. First, the outline of the two tasks will be described.

1) Reduction of corrosion of steel plate Due to moisture, salt, or corrosive gas content in crude oil, the oil tank is exposed to the corrosive environment (Japanese High Pressure Technical Association: Oil Tank Method and Corrosion Management Guidelines HPIS G, p .18 (1989-90), Japan Shipbuilding Association: H12 Research Summary Report, Study on New Corrosion Behavior of SR242 Crude Oil Tankers. In particular, oil tanker oil tanks contain volatile constituents, mixed seawater, salts in oilfield brine, and engine exhaust gases called inert gases, which are sent in oil tanks for explosion protection, and condensation due to temperature fluctuations during the day and night. In a unique corrosive environment, steel is damaged by total corrosion and formal local corrosion.

In the bottom plate of the oil tanker, a large number of pits about 100 to 30 mm in diameter occur. The speed of development reaches 2-3 mm / year. This is much higher than the average decay rate of 0.1 mm / year due to corrosion taken into account in hull design. In a crude oil tank, the local corrosion of the structural member is not particularly preferable for the following reasons and the like, so countermeasures are inevitable. If local corrosion progresses, the load on the part may unexpectedly increase, causing large warpage or plastic deformation, leading to destruction of the structure as a whole. In addition, the occurrence location and progress of local corrosion are difficult to predict. Therefore, it is expected to develop a steel having excellent strength and weldability as well as low corrosion resistance, in particular, a slow progress of local corrosion as a welded structural steel.

2) Reduction of solid sulfur deposited on the surface of steel sheet in the gas phase part causing sludge

In addition to the corrosion damage, a large amount of solid S is generated and precipitated on the inner surface of the steel tank, particularly on the surface of the steel plate on the back of the upper deck (deck plate). This is because iron rust on the surface of the corroded steel sheet becomes a catalyst and SO ₂ and H ₂ S in the gas phase react to form solid S. The formation of new iron rust and precipitation of solid S by the corrosion of the steel sheet alternately, and the layered corrosion products of iron rust and solid S precipitate. Since the solid S layer is soft, the product consisting of solid S and iron rust is easily peeled off and dropped off, and is deposited as sludge on the bottom of the oil tank. The amount of sludge to be recovered by regular inspection is said to be 300 tons or more in a super crude oil tanker, and the reduction of sludge mainly composed of solid S has been strongly demanded for maintenance.

As a technique for simultaneously reducing steel and solid sludge, the painting and lining methods are common, and a method of thermal spraying of zinc or aluminum is also proposed (Japan High Pressure Technical Association: Oil Tank). Corrosion and corrosion control guidelines in HPIS G, p. 18 (1989-90). However, in addition to the economic problems such as the construction period and cost, the repainting on the back of the deck plate of the super-large tanker is inevitably progressed due to minor defects and deterioration during construction of the anticorrosive layer. There were also problems in terms of technology, such as regular inspection and repair being inevitable.

Moreover, the technique which suppressed precipitation of solid S in the steel surface by improving the corrosion resistance of the steel itself in the crude oil tank environment is not disclosed. Therefore, in welded structure applications such as tanks, from the viewpoint of improving the reliability of the structure and extending the life, development of welded structural steel that is excellent in corrosion resistance and suppresses the formation of sludge mainly composed of solid S is expected.

Next, a description will be given of the proposed technology and the surrounding technology and the problems of the proposed technology in order to solve the problems 1) and 2).

1) Corrosion Reduction Countermeasures of Steel Sheet and Problems of Prior Art

Techniques proposed so far to mitigate corrosion, particularly local corrosion, of steel plates in the crude oil tank inner surface will be described. In crude oil tanks, it is common for crude oil tankers, ground or underground tanks to use ordinary steel for welding structures. Conventionally, the most common anticorrosive method is coating, and anticorrosive coating with an epoxy resin and / or zinc-rich primer, heavy anticorrosive coating with an epoxy resin containing glass flakes, and the like have been proposed. In addition, since hot dip galvanizing is applied because it is excellent in corrosion resistance in an environment of alternating contact with seawater and crude oil, it is used for handrails and piping of tankers. Moreover, the following techniques are proposed as corrosion-resistant steels which are excellent in corrosion resistance than ordinary steel and suitable for a crude oil tank inner surface use.

Japanese Patent Application Laid-open No. 50-158515 discloses an excellent corrosion resistance of Cu-Cr-Mo-Sb steel in an environment where crude oil and sea water are exposed alternately or at the same time as a downstream pipe. It is proposed by showing. The corrosion-resistant steel described in this patent is a steel containing Cr: 0.2 to 0.5% as a main component, Cu: 0.1 to 0.5%, Mo: 0.02 to 0.5%, and Sb: 0.01 to 0.1%.

Japanese Patent Application Laid-Open No. 2000-17381 proposes Cu-Mg steel as an anti-corrosion steel for shipbuilding, showing excellent corrosion resistance in use environments such as ship shells, ballast tanks, cargo oil tanks (crude oil tanks), and coal mine cargo hold. . Corrosion-resistant steel described in this patent is Cu: 0.01 to 2.0%, Mg: 0.0002 to 0.0150% as a main component, C: 0.01 to 0.25%, Si: 0.05 to 0.50%, Mn: 0.05 to 2.0%, P: 0.10% or less , S: 0.001 to 0.10%, Al: 0.005 to 0.10%.

Japanese Laid-Open Patent Publication No. 2001-107179 proposes that high P-Cu-Ni-Cr-high Al steel as corrosion resistant steel for oil tanks exhibits excellent corrosion resistance and weld cracking susceptibility on the back side of the deck plate of the oil tank. Corrosion-resistant steel described in this patent is P: 0.04-0.1%, S: 0.005% or less, Cu: 0.1-0.4%, Ni: 0.05-0.4%, Cr: 0.3-4%, Al: 0.2-0.8% as main components And C: 0.12% or less, Si: 1.5% or less, Mn: 0.2 to 3%, and satisfy Pcm≤0.22. However, Pcm = [% C] + [% Si] / 30 + [% Mn] / 20 + [% Cu] / 20 + [% Ni] / 60 + [% Cr] / 20 + [% Mo] / 15 + [% V] / 10 + 5 [% B].

Japanese Patent Laid-Open No. 2001-107180 discloses that corrosion resistant steel for low oil tanks and low P-Cu-Ni-Cr-high Al steel are subjected to excellent corrosion resistance and large heat input welding exceeding 100 kJ on the back of the deck plate of the lower oil tank. It is proposed that the balance of mechanical properties and weldability at the time exhibits excellent characteristics. The corrosion-resistant steel described in this patent is composed of P: 0.035% or less, S: 0.005% or less, Cu: 0.1 to 0.4%, Ni: 0.05 to 0.4%, Cr: 0.3 to 4%, and Al: 0.2 to 0.8% as main components. , C: 0.12% or less, Si: 1.5% or less, Mn: 0.2 to 3%, and satisfies Pcm? 0.22. However, Pcm = [% C] + [% Si] / 30 + [% Mn] / 20 + [% Cu] / 20 + [% Ni] / 60 + [% Cr] / 20 + [% Mo] / 15 + [% V] / 10 + 5 [% B].

Japanese Unexamined Patent Publication No. 2002-12940 discloses a corrosion resistant steel for an oil tank and a method of manufacturing the same, wherein Cu-containing steel, Cr-containing steel, and Ni-containing steel are introduced into a corrosive atmosphere at the top of the oil tank, that is, into the oil tank. Excellent corrosion resistance under primer coating against acid dew point corrosion environment caused by corrosive components in prime mover exhaust gas, and more particularly, durability to minimize rust progress under coating and consequently to extend coating life. It is proposed to show this excellent characteristic. The corrosion-resistant steel described in this patent is based on the premise of use in a primer coating state, and contains at least one of Cu: 0.1% to 1.4%, Cr: 0.2 to 4%, and Ni: 0.05 to 0.7% as a basic component. And C: 0.16% or less, Si: 1.5% or less, Mn: 3.0% or less, P: 0.035% or less, S: 0.1% or less, and at the same time, Pcm≤0.22. However, Pcm = [% C] + [% Si] / 30 + [% Mn] / 20 + [% Cu] / 20 + [% Ni] / 60 + [% Cr] / 20 + [% Mo] / 15 + [% V] / 10 + 5 [% B].

Japanese Patent Application Laid-Open No. 2003-105467 discloses a corrosion resistant steel sheet for oil tanks having excellent corrosion resistance at welded portions, wherein Cu-Ni steel has excellent corrosion resistance in the base metal used in the state of primer coating and the weld portion not subjected to primer coating. In addition, it is proposed that the conventional welding wire for carbon steel can be used at the same time. The corrosion-resistant steel described in this patent is based on Cu: 0.15% to 1.4% as a basic component, on the premise that it is used in a primer coating state, C: 0.16% or less, Si: 1.5% or less, Mn: 2.0% or less, P : It is a steel which contains 0.05% or less, S: 0.1% or less, and simultaneously satisfies Pcm≤0.24. However, Pcm = C + Si / 30 + Mn / 20 + Cr / 20 + Cu / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B.

Japanese Patent Application Laid-Open No. 2001-214236 discloses a corrosion resistant steel for crude oil and heavy oil storage, wherein Cu-containing steel, Cr-containing steel, Mo-containing steel, Ni-containing steel, Cr-containing steel, Sb-containing steel and Sn-containing steel are crude oil tankers and oil tanks. It is proposed to exhibit excellent corrosion resistance when storing liquid fuel and raw fuel such as crude oil and heavy oil. Corrosion-resistant steel described in this patent is any one of Cu: 0.01-2.0%, Ni: 0.01-7.0%, Cr: 0.01-10.0%, Mo: 0.01-4.0%, Sb: 0.01-0.3%, Sn: 0.01-0.3% Based on what contains 1 type, or 2 or more types, as a basic component, C: 0.003-0.30%, Si: 2.0% or less, Mn: 2.0% or less, Al: 0.10% or less, P: 0.050% or less, S: 0.050% It is a steel containing.

Japanese Patent Application Laid-Open No. 2002-173736 proposes that Cu-Ni-Cr steel exhibits excellent corrosion resistance as a corrosion resistant steel for oil tanks and storage tanks. The corrosion-resistant steel described in this patent is based on Cu: 0.5 to 1.5%, Ni: 0.5 to 3.0%, Cr: 0.5 to 2.0%, C: 0.001 to 0.20%, Si: 0.10 to 0.40%, and Mn: 0.50. It is steel containing -2.0%, P: 0.020% or less, S: 0.010% or less, and Al: 0.01-0.10%.

Japanese Laid-Open Patent Publication No. 2003-82435 discloses excellent corrosion resistance of Ni-containing steels and Cu-Ni steels for cargo oil tanks, and moreover, excellent corrosion resistance to wet and dry repeated corrosion containing inert gas. It is proposed to show. Corrosion-resistant steel described in this patent is Ni: 0.05 to 3% as a basic component, C: 0.01 to 0.3%, Si: 0.02 to 1%, Mn: 0.05 to 2%, P: 0.05% or less, S: 0.01% or less It is steel which contains 1 type, or 2 or more types of Mo, Cu, W, Ca, Ti, Nb, V, B, Sb, and Sn as needed.

In addition, the following techniques have been proposed for corrosion resistant steels proposed for marine ballast tanks but not for crude oil tankers.

Japanese Patent Publication No. 49-27709 proposes that corrosion-resistant low alloy steels exhibit excellent corrosion resistance in Cu-W steel and Cu-W-Mo steel in ballast tanks. Corrosion-resistant steel described in this patent is based on Cu: 0.15 to 0.50%, W: 0.05 to 0.5%, C: 0.2% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.1% or less It is steel which contains Mo: 0.05-1.0% as needed.

In Japanese Patent Laid-Open No. 48-509217, Patent Document 11 proposes that Cu-W steel and Cu-W-Mo steel exhibit excellent corrosion resistance in ballast tanks as corrosion-resistant low alloy steels. The corrosion-resistant steel described in this patent is based on Cu: 0.15-0.50%, W: 0.01-0.05% as a basic component, C: 0.2% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.1 It is steel which contains% or less and contains Mo: 0.05-1.0% as needed.

Japanese Patent Application Laid-Open No. 48-50922 discloses a steel ballast containing corrosion resistance low alloy steel containing Cu and W, and further containing one or two or more of Ge, Sn, Pb, As, Sb, Bi, Te or Be. It is proposed to exhibit excellent corrosion resistance in the tank, more particularly high resistance to local corrosion. Corrosion-resistant steel described in this patent is based on Cu: 0.15 to 0.50%, W: 0.05 to 0.5%, Ge, Sn, Pb, As, Sb, Bi, Te or Be, one or two or more: 0.010 to 0.2% It is steel containing C: 0.2% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.1% or less as needed, and Mo: 0.01 to 1.0% as needed.

Japanese Patent Application Laid-Open No. 49-3808 proposes that as a corrosion-resistant low alloy steel, Cu-Mo steel exhibits excellent corrosion resistance in a ballast tank and at the same time shows good strength characteristics and weldability. The corrosion-resistant steel described in this patent is based on Cu: 0.05 to 0.5%, Mo: 0.01 to 1%, C: 0.2% or less, Si: 1.0% or less, Mn: 0.3 to 3.0%, P: 0.1% or less It is a steel containing.

Japanese Patent Application Laid-Open No. 49-52117 discloses that seawater-resistant low alloy steels are excellent in corrosion resistance to seawater and, more particularly, in formulas or crevice corrosion that tend to occur in steels containing a large amount of alloying elements. It is proposed. Corrosion-resistant steel described in this patent is Cr: 1 to 6%, Al: 0.1 to 8% as a basic component, C: 0.08% or less, Si: 0.75% or less, Mn: 1% or less, P: 0.09% or less, S: Steel containing 0.09% or less.

Japanese Patent Application Laid-Open No. 7-310141 discloses a seawater resistant steel for high temperature and high humidity environment and a method of manufacturing Cr-Ti steel, which shows excellent seawater corrosion resistance in a high temperature and high humidity environment in ships, that is, ballast tanks or seawater piping, and at the same time, HAZ. It is proposed as a steel with excellent toughness. The corrosion-resistant steel described in this patent is a steel containing C: 0.1% or less, Si: 0.50% or less, Mn: 1.50% or less, and Al: 0.005-0.050% with Cr: 0.50 to 3.50% as a base component.

Japanese Patent Application Laid-open No. Hei 8-246048 discloses a method for producing a seawater resistant steel for high temperature and high humidity environments, which has excellent welded HAZ toughness, such as Cr-containing steel, high temperature and high humidity environments in ships, that is, excellent seawater corrosion resistance in ballast tanks and seawater piping. It is proposed to show. Corrosion-resistant steel described in this patent is based on Cr: 1.0 to 3.0%, Ti: 0.005 to 0.03%, C: 0.1% or less, Si: 0.10 to 0.80%, Mn: 1.50% or less, Al: 0.005 to 0.050% It is a steel containing.

Next, the subject of the said prior art is demonstrated.

In the case of reducing corrosion by coating of primer, heavy coating, and metal spray, etc., in addition to the problem of cost of construction, starting from the defects caused by minute defects or deterioration of the anticorrosive layer during construction Since local corrosion inevitably occurs and develops, there is a problem that corrosion progresses so much that there is no significant difference from the use as it is for 5 to 10 years in normal use. In addition, regular inspections and repairs are inevitable, resulting in problems such as maintenance costs. Moreover, about the local corrosion which generate | occur | produces in an oil bottom plate, there existed a problem that after the anticorrosive layer deteriorated, the progress rate of local corrosion was not significantly different from that used as it is.

In the oil pipes of Japanese Patent Application Laid-Open No. 50-158515, since the Cr contains more than 0.1% of Cr, which is harmful to corrosion resistance in a crude oil tank environment, the growth rate of local corrosion occurring in the bottom plate is not reduced, and the amount of alloy added. The problem was that the cost-effectiveness of the sum total could not be obtained from corrosion resistance. Moreover, since it contains Cr, there existed a subject that weldability is inferior compared with normal steel.

Since Mg addition is essential in shipbuilding corrosion resistant steel disclosed in Japanese Patent Application Laid-Open No. 2000-17381, the production stability of the steel is impaired. In addition, in the study of the present inventors, the corrosion of local corrosion occurring in the bottom plate in the Cu-Mg steel is studied. There existed a problem that the cost-effectiveness suitable for the sum total of the addition amount of an alloy cannot be obtained from corrosion resistance, without a progress rate decreasing.

In corrosion resistant steel (high P-Cu-Ni-Cr-high Al steel) described in Japanese Patent Application Laid-Open No. 2001-107179, Cr: 0.3-4% and Cr 0.1% that are harmful to corrosion resistance in an oil tank bottom plate environment Since it contains exceeding, the growth rate of the local corrosion which arises in a bottom plate does not reduce, and there existed a subject that the cost effect suitable for the sum total of the amount of alloy addition could not be obtained from corrosion resistance. Moreover, since Cr contained, there existed a subject that weldability was inferior compared with normal steel.

In corrosion resistant steel for low oil tanks (low P-Cu-Ni-Cr-high Al steel) described in Japanese Patent Application Laid-Open No. 2001-107180, Cr: 0.3-4% and Cr 0.1% that are harmful to corrosion resistance in crude oil bottom plate environments Since it contains exceeding, the growth rate of the local corrosion which arises in a bottom plate does not reduce, and there existed a subject that the cost effect suitable for the sum total of the amount of alloy addition could not be obtained from corrosion resistance. Moreover, since Cr contained, there existed a subject that weldability was inferior compared with normal steel. In addition, although under-film corrosion is suppressed in gas phase parts, such as the back of a deck plate in a primer state, since it contains comparatively large Cr and Al, the expansion width in a coating-film defect part is reduced, but it progresses to a plate thickness direction in a coating-film defect part. There was a problem that a reduction in the corrosion rate could not be achieved.

In corrosion resistant steel sheets (Cu-Ni steel) described in Japanese Patent Application Laid-Open Nos. 2002-12940 and 2003-105467, Cu and Ni have improved corrosion resistance, and more specifically, undercoat corrosion. It is effective for improving the resistance to, and Mo is said to be effective for improving the strength characteristics, although harmful to corrosion resistance. According to the examples, since all of the Cu-Ni-Mo steels proposed in the proposed corrosion resistant steels exceed the upper limit (0.2%) of Mo in the present invention, it suppresses the progress of local corrosion occurring in the crude oil bottom plate. There was a problem that no effect could be obtained.

Excellent corrosion resistance in crude oil and heavy oil storage steels (Cu-containing steel, Cr-containing steel, Mo-containing steel, Ni-containing steel, Cr-containing steel, Sb-containing steel and Sn-containing steel) described in Japanese Patent Application Laid-Open No. 2001-214236. In order to obtain, according to the Example, Cu: 0.22 to 1.2%, Cr: 0.3 to 5.6%, Ni: 0.5 to 6.2%, Mo: 0.25 to 7.56%, Sb: 0.07 to 0.25%, Sn: 0.07 to 1.5% Addition of species or two or more is inevitable, and the expression of the effect requires the addition of a large amount of alloying elements, and has a problem that it is inferior in economy and weldability.

In corrosion resistant steel (Cu-Ni-Cr steel) for crude oil transportation and storage tanks described in Japanese Patent Laid-Open No. 2002-173736, Cu: 0.5 to 1.5%, Ni: 0.5 to 3.0%, and Cr: 0.5 to 2.0 Since it contains%, the expression of the effect required the addition of a large amount of alloying elements, and had the problem that it was inferior in economy and weldability. Since Cr contains more than 0.1% of Cr which is harmful to corrosion resistance in a crude oil tank bottom environment, the rate of development of local corrosion occurring in the bottom plate is not reduced, and the cost effect suitable for the sum total of the amount of alloy addition cannot be obtained from corrosion resistance. There was a challenge.

In the steel materials for cargo oil tanks (Ni-containing steel, Cu-Ni steel) described in Japanese Patent Application Laid-Open No. 2003-82435, the development of local corrosion is suppressed in the corrosion test environment that simulates the back side of the deck plate rather than the oil bottom plate. We are examining the steel components. As steel without adding Cr and using Cu-Ni-Mo as a base component, Sample No. B4 (0.43% Cu-0.18% Ni-0.26% Mo) and B6 (0.33% Cu- in Table 4) described in this patent 0.31% Ni-0.35% Mo), B13 (0.38% Cu-0.12% Ni-0.44% Mo), B15 (0.35% Cu-0.28% Ni-0.31% Mo), B19 (0.59% Cu-0.16% Ni-0.22 % Mo) and B20 (0.59% Cu-0.44% Ni-0.22% Mo) correspond, but even any intensity | strength base component has such a required addition amount comparatively, and there existed a problem of cost and weldability. In addition, in order to obtain excellent puncture resistance in a crude oil tank bottom environment, Ni-containing steel or Cu-Ni steel is the basic component, and particle size exceeding 30 µm is less than 30 inclusions per cm 2, and pearlite in the metal structure ( There was a problem that a relationship of Ap / C ≦ 130 must be satisfied between the pearlite) ratio Ap and the amount of C in the steel.

Next, the problems of the corrosion resistant steel proposed for the use of ship ballast tanks will be described.

Corrosion-resistant low alloy steels (Cu-W steels and Cu-W-Mo steels) described in Japanese Patent Application Laid-open No. 49-27709 contain Al according to the chemical composition of the inventive steels shown in Example Table 1 of Patent Document 10. There was a problem that local corrosion resistance at the bottom of the crude oil tank could not be obtained. Moreover, it was not Al-kilted steel, and there existed a subject that it was difficult to apply it to the current shipbuilding steel from the viewpoint of steel cleanliness and weld part toughness.

Corrosion-resistant low alloy steels (Cu-W steels and Cu-W-Mo steels) described in Japanese Patent Application Laid-open No. 48-50921 contain Al according to the examples described in this patent and the chemical composition of the inventive steels shown in Table 1. There was a problem that local corrosion resistance at the bottom of the crude oil tank could not be obtained. Moreover, it is obvious that it is not Al-kilted steel, and it has been a problem that it is difficult to apply it to the current shipbuilding steel from the viewpoint of the cleanliness of steel and toughness of a weld part.

The corrosion-resistant low alloy steel described in Japanese Patent Application Laid-Open No. 48-50922 contains Cu: 0.15 to 0.50% and W: 0.05 to 0.5%, and is one of Ge, Sn, Pb, As, Sb, Bi, Te, or Be. Or 2 or more types: Since it is necessary to contain 0.01-0.2% further, there existed a subject that hot workability fell remarkably. Moreover, according to the chemical composition shown in Table 1 of this patent, since it does not contain Al, there existed a subject that local corrosion resistance in a crude oil tank bottom plate could not be obtained. Moreover, it is obvious that it is not Al-kilted steel, and there existed a subject that it was difficult to apply it to the current shipbuilding steel from the viewpoint of the cleanliness of steel and toughness of a weld part.

As a corrosion-resistant low alloy steel described in Japanese Patent Application Laid-open No. 49-3808, Cu-Mo steel is proposed as a corrosion resistant steel for ballast tanks, but according to the composition of the proposed steel proposed in the examples described in this patent, a desired ballast tank environment It is clear that S needs to contain 0.008% or more in order to obtain corrosion resistance at. Therefore, there existed a subject that local corrosion resistance in the crude oil tank bottom plate of the steel of this invention was not acquired. Moreover, since it does not contain Al, there existed a subject that local corrosion resistance in a crude oil tank bottom plate could not be obtained. Moreover, it is obvious that it is not Al-kilted steel, and there existed a subject that it was difficult to apply it to the current shipbuilding steel from the viewpoint of the cleanliness of steel and toughness of a weld part.

The corrosion resistant steels proposed in Japanese Patent Laid-Open No. 49-52117, Japanese Patent Laid-Open No. 7-310141, and Japanese Patent Laid-Open No. 8-246048 are based on steel containing 0.5% or more Cr. There was a problem that local corrosion resistance could not be obtained from the bottom of a crude oil tank.

In addition to the prior art described above, although several uses are described, some techniques of low alloy corrosion resistant steels are disclosed.

The member for automobile suspension device is accompanied by the attachment of a snow salt, and wet corrosion containing chloride ion arises. This corrosion problem is excellent in puncture resistance and is made of a low alloy steel for automobile suspension member, for example, by containing Cu, Ni, Ti and P in the steel, thereby producing a corrosion-resistant coating by phosphate on the steel surface. A technique characterized in that it is characterized by, for example, Japanese Patent Application Laid-Open No. 62-243738, or a technique in which rust protection is enhanced by adding P and Cu alone or in combination to steel to make the resulting rust layer amorphous and dense. (For example, Japanese Patent Application Laid-Open No. 2-22416). In addition, seawater-resistant low alloy steels having improved seawater resistance have been developed and marketed by various companies (for example, Matsushima Iowa, corrosion-resistant low-alloyed steel, p. 117, earthquake shockan, 1995).

However, in the case of steel or weather resistant steel which is excellent in puncture resistance for the automobile suspension device, the use environment forms a dense rust layer which is protected even in a soft environment, but such excellent puncture resistance is expressed when it is always wet. Instead, the proper wet and dry repetition is repeated to limit the environment in which the rust layer with dense protection is formed by itself, and the excellent puncture resistance is not exerted in a long use environment or a wet environment. In addition, in the case of the above-mentioned seawater-resistant low alloy steel, the characteristics of corrosion resistance evaluated at the average plate thickness reduction rate are often superior to that of ordinary steel, but it is clearly superior to that of ordinary steel in the local corrosion growth rate. There was a problem that it could not (Matsushima Iwao, corrosion-resistant low alloy steel, p. 112, earthquake shokan, 1995).

As described above, in the use of welded structures such as crude oil tanks, low alloy steels are expected to be developed at a slow rate of local corrosion even if full corrosion occurs from the viewpoint of improving the reliability of the structure and extending the service life. As a technique for reducing local corrosion progress occurring in a bottom plate of a crude oil tank, a method of anticorrosive lining of a bottom plate is currently proposed, and a ballast tank environment and a crude oil tank deck plate similar to a crude oil tank, which is the subject environment of the present invention, are proposed. Although many corrosion-resistant steels have been proposed so far to reduce corrosion occurring on the back side, the proposal of corrosion-resistant steels with slow progress of local corrosion occurring in crude oil bottom plate remains in the invention described in Japanese Patent Publication No. 2003-82435 described above. have.

2) Countermeasures of solid sulfur deposited on the surface of steel sheet in the gas phase part causing sludge and problems of the prior art

As a technique for simultaneously reducing steel and solid sludge, the coating and lining methods are common, and a method of thermal spraying of zinc or aluminum is also proposed (Japan High Pressure Technical Association). : Guidelines for corrosion protection and corrosion control of oil tanks HPIS G, p. 18 (1989-90)). However, as in the case of corrosion reduction measures, in addition to the economical problem of construction cost, corrosion is inevitably advanced due to minute defects and deterioration in the construction of the anticorrosive layer, so regular inspection is performed even after painting and lining. There was a problem that repair and maintenance were inevitable, and its life was limited to 5 to 10 years.

However, the technique of suppressing the precipitation of the solid S on the steel surface by improving the corrosion resistance of the steel itself in the crude oil tank environment is not disclosed. Therefore, in the use of welded structures, such as tanks, the development of the welded structural steel which is excellent in corrosion resistance from the viewpoint of the improvement of the structure reliability, and the life extension, and suppresses the formation of the sludge mainly from solid S was expected.

The present invention exhibits excellent corrosion resistance against crude oil corrosion generated in oil tankers of crude oil tankers or forced oil tanks for transporting or storing crude oil such as above-ground or underground crude oil tanks, and also contains a solid S. The present invention relates to a crude oil tank steel for welded structures capable of suppressing the formation of c), a method for producing the same, and a crude oil tank and an anticorrosive method thereof.

1 is a relationship between local corrosion growth rate and Mo content of Fe—Cu—Mo steel.

Fig. 2 is a relation diagram of local corrosion growth rate and Cu content of Fe—Cu—Mo steel.

3A is a diagram showing the relationship between the local corrosion growth rate and the P content of Fe—Cu—Mo steel.

3B is a diagram showing the relationship between the local corrosion growth rate and the S content of Fe—Cu—Mo steel.

4 is a relation diagram of local corrosion growth rate and Al content of Fe—Cu—Mo steel.

5 is a configuration diagram of a corrosion test apparatus.

6 is a diagram illustrating a temperature cycle added to a test piece.

The present invention has been made to solve such a problem, and its object is to show excellent local corrosion resistance in the bottom plate environment of a crude oil tank, and also to produce a slow rate of formation of a corrosion product containing solid S in the gas phase of the upper deck of the crude oil tank. The present invention provides a crude oil tank steel for welded structures, a method of manufacturing the same, and a crude oil tank and an anticorrosive method thereof.

In order to solve the above problems, the present inventors have obtained the following findings by examining the influence of the chemical composition, structure, and manufacturing method of steel on the local corrosion propagation behavior in the bottom plate of the crude oil tank and the precipitation behavior of the solid S on the back of the upper deck. .

[1] means for suppressing local corrosion progress in crude oil bottom plates

On the bottom of the crude oil tank, a large amount of salt water contained in the crude oil is separated and retained. The concentration of the brine depends on the yield of the crude oil and the oil well depth, but it was first found that it is approximately 1 to 60% by mass of concentrated brine in terms of NaCl. When the steel sheet is exposed to such rich brine, that is, the concentrated halogen solution, the surface of the steel sheet becomes uneven due to deposits such as corrosion products, sludge, and ash, and rapidly forms and fixes the site where iron is dissolved first. It was found that local corrosion progressed from those sites. In addition, since the pH buffering ability of the concentrated brine solution is extremely weak, at sites where iron is preferentially dissolved, the pH rapidly decreases to 2 or less due to hydrolysis of eluted iron ions or alloy ions, and localized starting from these sites. A mechanism is proposed in which corrosion accelerates catalytically.

In addition, the present inventors have studied the effects of Cu and Mo on the local corrosion growth rate, and Fe-Cu-Mo steels having various amounts of Cu (0.1 to 0.5% by mass) and Mo (0.025 to 0.075% by mass) that have been solvents in the laboratory. The following findings were obtained as a result of studying using

1 shows the influence of Mo addition amount on the local corrosion growth rate of Fe-Cu-Mo steel. It was found from FIG. 1 that the local corrosion growth rate took a minimum value near 0.05 mass% Mo, and reduced the inhibitory effect of Mo at 0.1 mass% or more. As a result, it was found that 0.03 to 0.07% is most preferable as the Mo addition amount.

2 shows the effect of the amount of Cu added on the local corrosion growth rate of Fe—Cu—Mo steel. From Fig. 2, it was found that the remarkable inhibitory effect of the local corrosion propagation rate by the addition of the Cu-Mo composite was remarkably recognized at Cu≥0.1 mass% and almost saturated at 0.3%.

3A and 3B show the effects of P and S on the local corrosion growth rate of 0.3% Cu-0.05% Mo steel. Impurities P and S showed a tendency to accelerate the local corrosion growth rate. When P contained more than 0.03%, and when S contained more than 0.02%, the local corrosion growth rate increased significantly. Moreover, when P <= 0.010% or S <= 0.0070% or less, it turned out that such an inhibitory effect can be minimized.

4 shows the effect of Al on the local corrosion growth rate of low p-low S-Cu-Mo steel. The curve of local corrosion growth rate shows a convex curve below, and when the amount of Al exceeds 0.3%, the local corrosion growth rate increases. It was found that when the Al was controlled at 0.01 to 0.1%, the local corrosion resistance was further improved.

Summarizing the above knowledge, their features,

(1) When Mo is added 0.01 to 0.1% by mass to a steel containing 0.1% by mass or more of Cu, the growth rate of local corrosion is remarkably lowered to 1/5 or less at ordinary steel ratio,

(2) When Mo is added in excess of 0.1% by mass to steel containing 0.1% by mass or more of Cu, the effect of inhibiting the growth rate of local corrosion by Mo is reduced;

③ The optimum Mo addition amount in the steel containing 0.1 mass% or more of Cu is 0.03-0.07 mass%,

④ The addition of excess P and S accelerates the local corrosion propagation rate and limits the upper limit of P and S so that excellent local corrosion resistance can be obtained.

⑤ When the amount of Al added is 0.01 to 0.1%, local corrosion resistance is further improved.

⑥ Cr is a harmful element that significantly accelerates local corrosion resistance, and is preferably limited to 0.01% or less,

Based on the findings obtained by the inventors of the present invention, it is possible to control the steel component of the low-alloyed steel to slow down the growth rate in the corroded portion after local corrosion.

Moreover, as a result of earnest research, the following knowledge was obtained.

That is, based on the chemical composition of the general welded structural steel, Cr is substantially added, one or both of a certain amount of Mo and W is combined with Cu, and the addition amount of impurities P and S is limited, and Al is added. By doing so, it was found that the following effects can be obtained.

1) By containing P, S, and Al in the limited range, the growth rate of local corrosion in the said environment is drastically reduced by the alloy addition amount of Cu, Mo, and W which are smaller.

2) As a result of studying the relationship between the presence state of Mo and W and the corrosion resistance in detail, the presence of Mo and W in solid solution is more preferable for corrosion resistance.

[2] measures for mitigating solid sulfur from the gas phase at the back of the crude oil tank top deck causing sludge

The present inventors earnestly studied the precipitation behavior of the solid sulfur in the gas phase on the steel plate surface of the crude oil tank upper deck, and obtained the following findings. (1) Solid S reacts and precipitates hydrogen sulfide and oxygen in an oil phase gas phase using an iron rust surface as a catalyst. (2) The precipitation rate of solid S depends not only on the temperature, the concentration of hydrogen sulfide and oxygen in the gas phase, but also on alloys containing trace amounts of iron rust. ③ If Cu and Mo are simultaneously included in iron rust, the precipitation rate of solid S is suppressed. (4) When Cu and Mo are included at the same time, the overall corrosion rate in the environment is also reduced. Based on the above knowledge, on the basis of the chemical composition of general welded structural steels, Cr is added without additives, a specific amount of Cu and Mo are added, and the amount of impurity P and S is limited to limit corrosion resistance in the environment, that is, It has been found that it is possible to improve the overall corrosion resistance.

This invention is mainly made | formed based on the said knowledge, The summary is as follows.

(1) In mass%, C: 0.001-0.2%, Si: 0.01-2.5%, Mn: 0.1-2%, P: 0.03% or less, S: 0.007% or less, Cu: 0.01-1.5%, Al: 0.001 -0.3%, N: 0.001-0.01%, Mo: 0.01-0.2%, W: 0.01-0.5% further, The remainder consists of Fe and an unavoidable impurity, It is characterized by the above-mentioned. Crude Oil Tanker Steel.

(2) The crude oil tanker steel according to the above (1), wherein the mass% is solid solution Mo + solid solution W ≥ 0.005%.

(3) The crude oil tanker steel according to the above (1) or (2), wherein the carbon equivalent (Ceq.) Represented by the formula (1) is at most% by mass.

Ceq. = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + W + V) / 5. Formula (1)

(4) Cr% is less than 0.1% by mass%, The crude oil tank steel according to any one of the above (1) to (3).

(5) Crude oil as described in any one of said (1)-(4) characterized by further containing 1 type or 2 types of Ni: 0.1-3% and Co: 0.1-3% by mass%. Quiet river.

(6) In mass%, one or two or more of Sb: 0.01 to 0.3%, Sn: 0.01 to 0.3%, Pb: 0.01 to 0.3%, As: 0.01 to 0.3%, and Bi: 0.01 to 0.3% are further added. Crude oil tanker steel in any one of said (1)-(5) characterized by including.

(7) In mass%, Nb: 0.002-0.2%, V: 0.005-0.5%, Ti: 0.002-0.2%, Ta: 0.95-0.5%, Zr: 0.005-0.5%, B: 0.0002-0.005% Crude oil tanker steel in any one of said (1)-(6) characterized by further containing 1 type (s) or 2 or more types.

(8) In mass%, one or two or more of Mg: 0.0001 to 0.01%, Ca: 0.0005 to 0.01%, Y: 0.0001 to 0.1%, La: 0.005 to 0.1%, and Ce: 0.005 to 0.1% are further added. The crude oil tanker steel according to any one of (1) to (7) above.

(9) The crude oil tanker steel according to any one of (1) to (8), wherein an area ratio of the micro segregation portion in which Mn is concentrated 1.2 times or more than the average Mn% of steel is 10% or less.

(10) It is a method of manufacturing the crude oil tanker steel in any one of said (1)-(9), and accelerates the steel piece which consists of a component in any one of said (1)-(8) after hot rolling. When cooling, the average cooling rate of accelerated cooling: 5 to 100 ° C / s, the accelerated cooling stop temperature: 600 ° C to 300 ° C, and the cooling rate to 100 ° C after the accelerated cooling stop is set to 0.1 to 4 ° C / s. A method for producing crude oil tank steel.

(11) A method for producing crude oil tank steel, comprising tempering or annealing the steel produced by the method described in the above (10) to 500 ° C or less.

(12) It is a method of manufacturing the crude oil tanker steel in any one of said (1)-(9), after hot-rolling the steel piece which consists of a component in any one of said (1)-(8), A method for producing a crude oil tanker steel, characterized in that, when produced by normalizing, the heating temperature of the casting is: Ac ₃ transformation point to 1000 ° C, and the average cooling rate of 700 to 300 ° C is 0.5 to 4 ° C / s.

(13) A method for producing crude oil tank steel, characterized in that it is subjected to annealing or annealing at 500 ° C or lower after the annealing of the above (12) substrate.

(14) Diffusion heat treatment of heating temperature: 1200-1350 degreeC and storage time: 2-100 hours is performed before hot-rolling the steel piece which consists of a component as described in any one of said (1)-(8). The manufacturing method of the crude oil tank steel in any one of said (10)-(13) mentioned above.

(15) A crude oil tank, wherein part or all of the bottom plate, the deck plate, the side plate, and the aggregate are made of the crude oil tank steel according to any one of the above (1) to (9).

(16) A method for producing a crude oil tank, wherein the hot rolled scale on the surface of the crude oil tank according to the above (15) is mechanically or chemically removed to expose an iron raw material.

(17) The anticorrosive method for crude oil tank according to the above (16), wherein after the mechanically or chemically removing the hot rolled scale, one or more layers of a coating film having a thickness of 100 µm or more are formed.

The present invention overcomes the above problems and achieves the object, and specific target means thereof will be described below.

First, the component element concerning this invention and its content are demonstrated. The unit of% of component content shown in description is mass%.

De-C to less than 0.001% is contained in an amount of 0.001% or more because industrially remarkably impairs economic efficiency, but when used as a reinforcing element, the content of 0.002% or more is more preferable. On the other hand, when it contains exceeding 0.2% excessively, weldability, deterioration of joint toughness, etc. generate | occur | produce, and since it is not preferable as steel for welded structures, 0.001 to 0.2% was made into the limited range. From the viewpoint of weldability, C is more preferably 0.18% or less. In particular, mild steel (yield stress of 240 N / mm 2 class) and high tensile steel (yield stress of 265,315,355,390 N / mm 2 class) for ship use and 0.05 to 0.15% of high tensile steel ship steel sheet are more preferable. C is an element which slightly reduces the local corrosion resistance in the crude oil tank bottom plate, and 0.15% or less is preferable from the viewpoint of corrosion resistance.

Si is required as the deoxidation element, and 0.01% or more is required to exhibit the deoxidation effect. Si is effective in improving the overall corrosion resistance and is a small but effective element in improving the local corrosion resistance. In order to express the effect, it is preferable to contain 0.1% or more. On the other hand, excessively containing Si causes fixation (degradation of scale peelability) of the hot rolled scale and increases the defects caused by the scale, so that the upper limit is 2.5% in the present invention. In particular, in the case of steel with a high demand for weldability, a base material and joint toughness as well as corrosion resistance, the upper limit is preferably 0.5%.

Mn is required 0.1% or more to secure the strength of the steel. On the other hand, when it exceeds 2%, deterioration of weldability and grain boundary embrittlement sensitivity are not preferable, and therefore, the range of Mn is limited to 0.1 to 2% in the present invention. In addition, since C and Mn are elements which hardly affect the corrosion resistance, it is possible to adjust the amount of C and Mn especially when limiting the carbon equivalent in the use of welded structures.

P is an impurity element, and when it exceeds 0.03%, it accelerates the local corrosion growth rate and deteriorates the weldability, so it is limited to 0.03% or less. In particular, when the content is 0.015% or less, 0.015% or less is preferable because it has a good effect on corrosion resistance and weldability. Moreover, although manufacturing cost increases, since corrosion resistance further improves, it is more preferable to make P into 0.005% or less.

S is also an impurity element, and if it exceeds 0.007%, it tends to accelerate the local corrosion growth rate and increase the amount of sludge production. In addition, mechanical properties, in particular, ductility is remarkably deteriorated, so the upper limit is 0.007%. The amount of S is so preferable that it is small with respect to corrosion resistance and mechanical properties, and 0.005% or less is especially preferable.

When Cu and 0.01% or more are contained together with Mo and W, it is effective for not only whole surface corrosion resistance but also local corrosion resistance improvement. Moreover, when it adds 0.03% or more, it is effective also in suppressing formation of solid S. If the content exceeds 1.5%, the adverse effects such as the growth of the surface cracks of the steel piece and the deterioration of the joint toughness are also present, so the upper limit is 1.5% in the present invention. Since the improvement of corrosion resistance is almost saturated even if it adds more than 0.5%, when suppressing the development of local corrosion of a crude oil bottom plate, 0.01 to 0.5% is preferable. Since the sludge formation inhibitory effect is almost saturated when 0.2% or more is added, when it is applied to a crude oil tank upper deck, it is more preferable that it is less than 0.03 to 0.2% for balance of productability.

Al, when added together with Cu and Mo and / or W, is an inevitable element in suppressing the progress of local corrosion. Moreover, it is an element effective for refine | miniaturizing the heating austenite particle diameter of a base material by AlN. Moreover, it also has the effect of suppressing the production of the corrosion product containing solid S, which is advantageous. However, in order to exhibit these effects, it is necessary to contain 0.001% or more. On the other hand, when it contains excessively more than 0.3%, since it forms a rough and large oxide and degrades ductility and toughness, it is necessary to limit to 0.001%-0.3% of range. In order to acquire sufficient corrosion resistance improvement effect and the effect which suppresses the formation of the corrosion product containing solid S, addition of 0.02% or more is more preferable. Since the corrosion resistance improvement effect is almost saturated even if it adds exceeding 0.1%, 0.02-0.10% is more preferable.

N is not preferable because it adversely affects ductility and toughness in solid solution. However, N is effectively used for improving austenite fineness and precipitation strengthening in combination with V, Al, and Ti. In addition, it is impossible to completely remove N in steel industrially, and it is not preferable to reduce more than necessary because it puts an excessive load on a manufacturing process. Therefore, the lower limit is set to 0.001% as a range in which the adverse effects on ductility and toughness can be allowed and also industrially controlled and the load on the manufacturing process can be allowed. Although N has the effect of slightly improving the corrosion resistance, since excessively contained N may increase the solid solution N and adversely affect the ductility and toughness, the upper limit is made 0.01% as an acceptable range.

Mo and W are important elements similar to Cu with respect to local corrosion characteristics, and by containing them together with 0.01% or more of Cu, it exhibits a remarkable effect especially in reducing the local corrosion growth rate. Mo and W have almost the same effect, and it is necessary to contain Mo or W individually or both in 0.01 to 0.2% and W in 0.01 to 0.5%, respectively. When 0.01% or more of Mo and 0.01% or more of W are contained, a definite effect is exerted on the improvement of local corrosion resistance. On the other hand, when Mo exceeds 0.2% and W contains more than 0.5%, local corrosion resistance is lowered and the weldability and toughness deteriorate. Therefore, Mo is limited to 0.01 to 0.2% and W is 0.01 to 0.5%. Moreover, in order to suppress formation of a precipitate and to ensure solid solution Mo and W reliably, it is more preferable to make upper limits of Mo and W into 0.1% and less than 0.05%, respectively. In addition, when 0.01 to 0.08% of Mo is added, significant improvement in local corrosion resistance can be obtained with a small amount of addition, and therefore 0.01 to 0.08% is more preferable. Moreover, when manufacturing stability is considered, 0.03-0.07% is more preferable. Moreover, when W is less than 0.01 to 0.05%, since the remarkable improvement of local corrosion resistance can be obtained with a small addition amount, less than 0.01 to 0.05% is more preferable.

Although the range of said Mo and W is a necessary condition, in order to exhibit the effect of local corrosion resistance more effectively, it is necessary to ensure high capacity of Mo and W more than fixed after making content into the said range. That is, when Mo and W form coarse precipitates, the depleted layer of the element is formed around them, so that the local corrosion resistance improving effect is impaired. Therefore, Mo and W need to exist uniformly. Since Mo and W in solid solution have an equivalent effect on the local corrosion resistance, if the sum of the high capacities of both elements is 0.005% or more, the local corrosion resistance is greatly improved. Although the upper limit of a high capacity | capacitance can be acquired without specifying especially, in order to obtain moderate strength economically because an intensity | strength rises by solid solution strengthening, it is preferable that the upper limit of the high capacity of both elements shall be 0.5% or less.

In addition, solid solution Mo and solid solution W which are effective for local corrosion resistance improvement in this invention refer to the quantity which deducted the precipitation amount calculated | required by extraction residue analysis from the total content. In other words, in the extraction residue analysis, extremely fine precipitates, which are considered to be solid solution, can be considered to exist in the steel uniformly according to the solid state.

Although the above is a basic requirement regarding the chemical composition in the steel of this invention and the reason for its limitation, in this invention, the limitation regarding the element which may be selectively added for the purpose of improvement of various characteristics etc. is made.

First, when it is necessary to consider weldability and weld joint toughness specially, carbon equivalent (Ceq.) Shown by Formula (1) shall be 0.4% or less.

Ceq. = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + W + V) / 5. Formula (1)

Equation (1) is a carbon equivalent formula including W, which is also an important element in the steel of the present invention. When the carbon equivalent of formula (1) is 0.4% or less, hardening of the heat affected zone by welding is suppressed, and the low temperature crack resistance In order to reliably improve the toughness of the weld heat affected zone (HAZ), 0.4% or less is preferable. When the carbon equivalent of Formula (1) exceeds 0.4% and excessively, depending on a combination of components, there exists a possibility that it may cause deterioration of low temperature crack resistance, HAZ toughness, and deterioration of the corresponding force corrosion cracking characteristic of HAZ. Although the lower limit of a carbon equivalent is not specifically determined, the effect of this invention can be acquired, but in order to acquire the outstanding toughness in the low temperature range of 0-40 degreeC, it is preferable to set the minimum of carbon equivalent to 0.36%.

Cr may be added as necessary to adjust the strength as a reinforcing element, but since Cr is an element which accelerates the local corrosion propagation rate most, the smaller the content, the more preferable the content of Cr is 0.1% or more. Deterioration and at the same time slightly promote the production of solid S. Therefore, it is not preferable to contain 0.1% or more in this invention. Therefore, less than 0.1% is preferable even if it does not intentionally contain, or inescapably or intentionally contains.

Ni and Co are effective elements for improving the base metal and the HAZ toughness, and are effective for improving the corrosion resistance and suppressing the sludge in steels containing Cu and Mo. By containing 0.1% or more of both elements, the toughness improvement and the corrosion resistance improvement effect are expressed clearly. On the other hand, the excess content of both elements in excess of 3% causes both elements to be economically inadequate as expensive elements and leads to deterioration of weldability. Therefore, in the present invention, when both Ni and Co are contained, 0.1 to 3 The content is limited to%.

Since 0.01% or more of Sb, Sn, As, Bi, and Pb each have the effect of further suppressing the progress of local corrosion, the lower limit when containing it as necessary is 0.01%, but exceeds 0.3%, respectively. Even if it contains excessively, the effect will be saturated, and there exists a possibility of the bad influence to another characteristic, and also takes into consideration economical efficiency, and an upper limit is made into 0.3%. 0.01 to 0.15% are more preferable.

Nb, V, Ti, Ta, Zr, and B are effective elements for increasing the strength of the steel in small amounts, and are mainly contained as necessary for the strength adjustment. In order to express an effect, it is necessary to contain Nb 0.002% or more, V 0.005% or more, Ti 0.002% or more, Ta 0.005% or more, Zr 0.005% or more, and B02 0.0002% or more. On the other hand, Nb is more than 0.2%, V is more than 0.5%, Ti is more than 0.2%, Ta is more than 0.5%, Zr is more than 0.5%, and B is more than 0.005%, which is not preferable because toughness deterioration is remarkable. Therefore, when Nb, V, Ti, Ta, Zr, and B are contained as necessary, Nb is 0.002-0.2%, V is 0.005-0.5%, Ti is 0.002-0.2%, Ta is 0.005-0.5%, Zr Is 0.005 to 0.5%, and B is limited to 0.0002 to 0.005%.

Mg, Ca, Y, La, and Ce are effective for the shape control of inclusions, and are effective for improving the ductility characteristics, and are also effective for improving the HAZ toughness of high heat input welded joints, and also the effect of suppressing sludge formation by fixing S. Because it is weak, it is included if necessary. As for content of each element in this invention, a lower limit is determined from the lower limit which an effect expresses, and Mg, 0.0001%, Ca are 0.0005%, Y is 0.0001%, La is 0.005%, Ce is 0.005% as a lower limit, respectively. . On the other hand, the upper limit is determined by whether the inclusions are largely roughened and adversely affect the mechanical properties, particularly ductility and toughness. In the present invention, the upper limit is Mg, Ca of 0.01%, Y, La, and Ce of 0.1%. do. When 0.005% or more of Mg and Ca are added, the effect of suppressing acidification in locally corroded food spaces is further expressed, and therefore 0.0005% to 0.1% is more preferable.

Although the above is the limitation reason regarding the chemical composition in this invention, Furthermore, in this invention, the micro segregation state of steel is also prescribed | regulated as needed according to the property of a steel piece. That is, in order to express local corrosion, the element which expresses local corrosion resistance needs to be distributed uniformly in steel. For that purpose, it is preferable that the degree of micro segregation is small. Moreover, if there exists a fluctuation | variation of the concentration of a component element other than a local corrosion-resistant expression element, it only promotes local corrosion. Therefore, in this invention, a micro segregation state is also limited as needed. Since the micro segregation state can be almost represented in the segregation state of Mn, in the present invention, when defining the micro segregation state, the area ratio of the micro segregation portion in which Mn is concentrated 1.2 times or more than the average Mn% of steel is 10%. It is set as follows.

The reason for limiting the micro segregation state as described above is that the difference in concentration with the secondary segregation becomes insignificant from the viewpoint of corrosion resistance when the element concentration is significantly concentrated by more than 1.2 times the average. Based on the experiment, it was confirmed that the proportion of the enriched region was not more than 10% by the area ratio in the cross section, so that it was not substantially adversely affected. In the present invention, the concentration of Mn was 1.2 times or more than the average Mn% of the steel. The area ratio of the micro segregation part to be made shall be 10% or less. The smaller the lower limit of the area ratio of the micro segregation portion, the more preferable, and 0% is optimal.

In addition, measurement of micro segregation is performed by an X-ray microanalyzer, and the area ratio of the area | region where Mn concentration becomes 1.2 times or more of average Mn concentration in a density | concentration map is calculated | required. The measurement is carried out on a plate thickness cross section perpendicular to the steel surface, measuring several places in the plate thickness direction from directly below the surface to 1/2 of the plate thickness from the surface of the steel to meet the requirements of the present invention at each position. There is a need.

Next, the requirements of the present invention for the steel production method for controlling the micro segregation state in order to secure the above-described requirements of the steel of the present invention, mainly the solid solution Mo, W, will be described below. However, the requirement regarding the steel of this invention does not have a problem with the means of achieving it. That is, it is not limited to the manufacturing method of this invention.

In the present invention, there are mainly two types of manufacturing methods for securing high capacities of Mo and W, which are largely divided into the case of producing by ① heat treatment and ② by producing by hot rolling after hot rolling. Moreover, as a control method of micro segregation, it is common to the method (1) and (2), and it is a requirement to perform (3) diffusion heat treatment before hot rolling. The requirements are summarized below.

① When manufactured by the work heat treatment which performs accelerated cooling after hot rolling, cooling to the average cooling rate of accelerated cooling is 5-100 degreeC / s, accelerated cooling stop temperature 600-30300 degreeC, and accelerated cooling stop to 100 degreeC. The speed is 0.1 to 4 ° C / s, and after completion of hot rolling and acceleration cooling, it is soaked or annealed to 500 ° C or less as necessary.

(2) When manufactured by hot-rolling after hot rolling, the heating temperature of the hot-rolling is an average cooling rate of 0.5-3 DEG C / s at Ac ₃ transformation point-1000 DEG C and 700-300 DEG C. Carry out annealing or annealing.

(3) Before hot rolling, diffusion heat treatment is performed with a heating temperature of 1200 to 1350 ° C and a storage time of 2 to 100 hours.

First, the method of ① will be described.

When manufacturing by the process heat processing which accelerates cooling after hot rolling, in order to ensure the required amount of solid solution Mo and W, it is first necessary to define cooling conditions including accelerated cooling after hot rolling.

Although accelerated cooling is performed by water cooling etc., the average cooling rate of accelerated cooling is 5-10 degreeC / s, the stop temperature of the accelerated cooling is 600-300 degreeC, and cooling after an accelerated cooling stop is 0.1- to the accelerated cooling stop-100 degreeC. It is necessary to cool at 4 ° C / s.

The lower limit of the cooling rate of the accelerated cooling is 5 ° C / s because the improvement of the strength and toughness due to the accelerated cooling is not clear if the cooling rate is less than 5 ° C / s. It is because there exists a possibility that Mo and W cannot form a precipitate during cooling and cannot secure solid solution Mo and W. On the other hand, the larger the cooling rate of the accelerated cooling is, the more preferable for the improvement of the strength and the suppression of precipitation of Mo and W. However, if the temperature exceeds 100 ° C / s, the effect on them is saturated and the shape of the steel sheet may deteriorate. In order to increase, the upper limit is made 100 ° C / s.

Accelerated cooling stops in the range of 600-300 degreeC. If the stop of the accelerated cooling exceeds 600 ° C, even if the cooling rate after the accelerated cooling stop is within the scope of the present invention, Mo and W form precipitates after the accelerated cooling stop, and the amount of solid solution Mo and W is not sufficiently secured. It is not preferable because the corrosion resistance may be slightly impaired as compared with the case of securing the amount specified in the present invention. On the other hand, if the accelerated cooling stop temperature is less than 300 ° C, it is preferable to secure the toughness level required especially for steel for welded structures, depending on the chemical composition, and also because the residual stress is large and the shape of the steel is deteriorated. Not. In addition, the start temperature of accelerated cooling does not need to be particularly defined since the influence on the amount of solid solution Mo and W is very small compared to the accelerated cooling stop temperature. However, in order not to deteriorate the strength and toughness, the start temperature of the accelerated cooling should be started immediately. It is preferable. Based on starting above the Ar ₃ transformation point, no particular problem is caused.

In addition, in order to ensure the solid solution Mo and W amount, it is also necessary to consider cooling after acceleration cooling stop. That is, when cooling from accelerated cooling stop to 100 ° C becomes slow cooling of less than 0.1 ° C / s, there is a possibility that Mo and W form carbonitrides during the cooling. Therefore, for example, when the thickness of the steel is wide and the cooling rate cannot be lowered to less than 0.1 C / s in air cooling, the cooling rate is 0.1 C / s or more by means of shower cooling or gas cooling. It needs to be controlled as possible. The larger the cooling rate, the more effective the effect of securing the solid solution Mo and W, but the effect is saturated above 4 ℃ / s, while the difference from the accelerated cooling after hot rolling controlled at 5 to 100 ℃ / s is not clear. In addition, since there exists a possibility that deterioration of toughness, an increase in residual stress, etc. may become present, 4 degree-C / s is made into an upper limit in this invention.

In order to make the above hot rolling and cooling process into a final process or to adjust the material, annealing or annealing may be further performed.However, in order to suppress the precipitation of Mo and W in the annealing or annealing and to secure the amount of solid solution Mo and W, It is necessary to limit the temperature of soaking or annealing to 500 degrees C or less.

Next, the method of ② will be described.

The method of (2) is the method of the present invention in the case of producing steel by casting. As in the method of (1), in order to suppress precipitation of Mo and W and to secure a required amount of solid solution Mo and W in the step of the step of casting, it is necessary to define various conditions for the step of casting. In addition, at the time of austenite single phase in the heating step of the collimation, the influence of the hysteresis so far is eliminated, so the conditions of hot rolling prior to the collimation do not matter. Therefore, hot rolling may be normal rolling which continuously rolls, control rolling, or the processing heat processing with accelerated cooling may be sufficient. In addition, the history before and after the hot rolling need not be particularly limited.

The basic requirement of the method of (2) is that after the hot rolling, when produced by the casting, the heating temperature of the casting is set at Ac ₃ transformation point to 1000 ° C, and the average cooling rate of 700 to 300 ° C in the cooling process is 0.5 to 4 ° C /. It is in doing with s.

If the heating temperature is less than the Ac ₃ transformation point, the corrosion resistance is deteriorated because Mo and W, which have been precipitated before casting, cannot be sufficiently dissolved. In addition, since the structure becomes nonuniform, deterioration in strength and toughness also occurs, which is not preferable. If the heating temperature is more than 1000 ° C, the heating austenite becomes coarse, resulting in coarsening of the final metamorphic structure, leading to marked deterioration in toughness, which is undesirable. Therefore, in the present invention, the heating temperature of the Ac ₃ transformation point to sojun ~1000 ℃.

Normally, after heating and preservation, the cooling is performed by air cooling in the case of casting, but in the present invention, when cooling is required to secure solid solution Mo and W, and in case of excessive cooling by air cooling, the means may be used. It is necessary to control and to set the average cooling rate of 700-300 degreeC to 0.5-4 degreeC / s. If the average cooling rate in 700-300 degreeC is less than 0.5 degreeC / s, there exists a possibility that Mo and W will form a precipitate during cooling, and cannot secure the solid solution Mo and W amount of the range of this invention. The larger the cooling rate is, the more effective the effect of securing the solid solution Mo and W is. However, the effect is saturated only above 4 ° C / s, and since the adverse effects such as deterioration of toughness and increase of residual stress may be present, the present invention is expected. In 4 ° C / s, the upper limit is set. In the collimation, since it does not involve accelerated cooling as in the method of (1), a cooling rate of less than 300 ° C. is not particularly relevant, but a slow cooling of 0.1 ° C./s or less at an average cooling rate of 300 to 100 ° C. is preferable. Not.

Although the above-mentioned annealing process may be used as the final process, or further, annealing or annealing may be performed for material adjustment, but in order to secure the amount of solid solution Mo and W by suppressing the precipitation of Mo and W in the annealing or annealing, It is necessary to limit temperature to 500 degrees C or less.

Finally, the method of ③ is explained. Method 3 is one means for satisfying the requirements of the present invention regarding micro segregation. The basic requirement is diffusion heat treatment with a heating temperature of 1200 to 1350 ° C. and a storage time of 2 to 100 h in that temperature range before hot rolling. It is in carrying out. The element segregated to micro by diffusion heat treatment diffuses to reduce the concentration of the micro segregated portion. In the diffusion heat treatment, if the heating temperature is less than 120 ° C., the diffusion rate of the element becomes too small, and a sufficient diffusion effect cannot be obtained in practical storage time. The higher the heating temperature, the faster the diffusion rate, which is advantageous for reducing segregation. However, the heating austenite grain size becomes excessively coarse, and the coarse structure remains after the subsequent hot rolling or heat treatment, which may adversely affect the mechanical properties. In addition, the possibility of causing roughness of the steel surface increases, which is not preferable. In this invention, the upper limit of a heating temperature shall be 1350 degreeC from a viewpoint which these bad influences can accept practically.

When the heating temperature of the diffusion heat treatment is set to 1200 to 1350 ° C, the storage time is required for 2 hours or more in order to sufficiently reduce micro segregation. The longer the storage time is, the more the diffusion proceeds. However, in the case of premature micro-segregation of ingots or slabs, a sufficient diffusion heat treatment effect can be obtained after 100 hours of storage. The upper limit is 100 hours.

Cooling after storing for 2 to 100 hours at 1200 to 1350 ° C. is not particularly a problem. However, when cooling effect is also expected, cooling is preferably cooled by air or less.

In addition, in the present invention, the size of the steel is increased after hot rolling, and practically, the diffusion heat treatment after the hot rolling is likely to be a problem in the ability of the heat treatment furnace, and the necessity of refining the structure once coarsened by the diffusion heat treatment is required. The diffusion heat treatment is performed before the hot rolling. However, in the method (2) of the present invention, if the problem is not solved, the effect does not decrease even if the diffusion heat treatment is performed after hot rolling and before the casting.

Next, the crude oil tank which consists of this invention steel is demonstrated. By using the steel of the present invention for a part or all of the bottom plate, deck plate, ceiling plate, side plate, and aggregate of the crude oil tank, it is possible to make the progress of local corrosion occurring in the crude oil tank extremely small, thereby reducing the frequency of repair of the crude oil tank, Improvement of safety is aimed at. Below, the effect of the crude oil tank using the steel of this invention is demonstrated in detail compared with the crude oil tank using a normal steel.

Rich brine contained in crude oil is separated to the bottom, causing local corrosion in various parts of the tank. In particular, local corrosion is inevitable on the bottom plate or side surfaces. By using the steel of the present invention in the site where the local corrosion occurs or the whole oil tank according to the oil tank structure, the local corrosion growth rate of the crude oil tank is significantly lowered. In particular, by using the steel of the present invention selectively at sites exposed to the concentrated brine without washing due to structural problems, it is possible to make the oil tank excellent in durability and economical.

In general, crude oil tankers inspect the position and depth of local corrosion at regular release inspections, and maintenance by reinforcement welding or the like is obligated for formulas having a predetermined depth or more. Therefore, in the crude oil tank using the steel of the present invention, when the regular inspection period is a certain interval, the number of formulas to be repaired is overwhelmingly reduced, and the cost and time required for repair can be greatly reduced. In addition, even if the growth of local corrosion is not repaired due to missing inspection, if the plate thickness is the same as that of crude oil tank using ordinary steel, the probability of penetration through local corrosion and leakage of crude oil is lowered. Contribute to improvement By using the steel of the present invention, the crude oil tank having excellent economic and safety aspects can be obtained in the same weldability and mechanical properties as in the case of using ordinary steel. In addition, by using the steel of the present invention for the deck plate and the ceiling plate, it is possible to greatly suppress the production of sludge on the back of the deck and the back of the ceiling plate, and it is also possible to reduce the cost of sludge recovery.

Below, the effect of this invention is demonstrated in detail by an Example. In addition, this invention is not limited to an Example as follows.

<Example>

The starting steel was melted by vacuum melting or converter to produce an ingot or steel piece on a steel sheet. Table 1 shows the chemical composition and Table 2 shows the conditions for producing the steel sheet. The manufacture of steel sheet changes conditions and a combination, respectively, by diffusive heat processing, hot rolling, annealing, and so that the effect of the manufacturing method of this invention can be made clear. In addition, in Table 2, the measurement result of the micro segregation state of the solid solution Mo, W amount, and Mn of the steel plate which started was shown together. Solid solution Mo and W amount were calculated | required by extraction residue analysis about the steel plate total thickness sample from which black skin was removed. The measurement of micro segregation was carried out by an X-ray microanalyzer at each position 1 mm below the surface of the cross section perpendicular to the surface of the steel sheet, at the 1/4 position of the plate thickness, at the center of the plate thickness, and at each position, and the Mn concentration was averaged in the concentration map. The area ratio of the area which becomes 1.2 times or more of Mn concentration was calculated | required by image analysis.

Table 3 shows the mechanical properties (strength, 2mmV notch Charpy impact characteristics) and the maximum hardness of the weld heat affected zone as weldability. Tables 4 and 5 show the corrosion resistance test results. In addition, Table 4 is a test mainly for evaluating local corrosion resistance, and Table 5 is a test mainly for evaluating the overall corrosion resistance and sludge formation behavior.

As the mechanical properties of the steel sheet, the strength and toughness were examined by a round bar tensile test and a 2 mmV notch Charpy impact test, but the test piece was taken from the plate thickness center in a direction in which the test piece longitudinal direction was perpendicular to the rolling report. The tensile test was conducted at room temperature, and the 2 mmV Notch Charpy Impact test was conducted at various temperatures, and the wavefront transition temperature obtained from the transition curve was used as an index of toughness.

The highest hardness test of the weld heat affected zone was carried out under the condition of not preheating according to JIS Z 3101.

The test conditions mainly for evaluating local corrosion resistance of Table 4 are as follows.

The test piece of length 40mm, length 40mm, and thickness 4mm was extract | collected so that the plate thickness quarter position of a steel plate might become the thickness center of a test piece. The whole surface of the test piece was machine-grinded, and after 600 wet grinding, the cross section was coat | covered with paint, leaving the front and back surface of 40 mm x 40 mm. The test piece was immersed in two kinds of corrosion solutions of 20 mass% NaCl aqueous solution which adjusted pH to 0.2 with hydrochloric acid. Immersion conditions were implemented at the liquid temperature of 30 degreeC, and immersion time for 24 hours-4 weeks, the corrosion loss was measured, and the corrosion rate was evaluated. The corrosive composition simulates the environmental conditions when local corrosion occurs in the actual steel structure, and the progress of local corrosion in the real environment is reduced by decreasing the corrosion rate in the corrosion test.

The test conditions for investigating the corrosive and sludge formation behavior of Table 5 are as follows.

The test piece of length 40mm, length 40mm, and thickness 4mm was extract | collected so that the plate thickness quarter position of a steel plate might become the thickness center of a test piece. The whole surface of the test piece was machine-grinded, and after 600 wet grinding, the back side and the cross section were coat | covered with paint, leaving the surface of 40 mm x 40 mm. The corrosion rate of the starting steel and the production rate of sludge mainly composed of solid S were evaluated using the test apparatus shown in FIG. Table 6 shows the composition of the gas used in the corrosion test.

The gas was sent to the test chamber 3 after adjusting to a constant dew point (30 degreeC) through the dew point adjustment tank 2. NaCl aqueous solution was apply | coated and dried on the surface of the test piece 4 so that the adhesion amount of NaCl might be 1000 mg / m <2> before a corrosion test, and it installed horizontally in the constant temperature heater board 5 in a test chamber. By controlling the heater controller 6, as shown in Fig. 6, a temperature cycle of 2 hours / cycle in total of 20 deg. C x 1 hour and 40 deg. C x 1 hour was given, so that wet and dry repetition occurred on the surface of the test piece. After 720 cycles, the corrosion rate was evaluated from the loss of corrosion, and the sludge formation rate was evaluated from the amount of material produced on the test piece surface. In addition, the product was confirmed by preliminary tests by chemical analysis and X-ray analysis, and iron oxyhydroxide (iron rust) and solid S.

In Examples, it is evident from the results in Table 3 that the steels of steel sheets Nos. A1 to A26 that satisfy the requirements of the present invention with respect to mechanical properties first have sufficient characteristics as weld structural steels. In terms of weldability, in the steel sheet of the present invention in which the carbon equivalent represented by the formula (1) was 0.4% or less, the maximum hardness of the weld heat-affected portion was reliably 300 or less in Vickers hardness, and it is clear that it had good weldability. .

Moreover, although steel plate number A25 is a range example of this invention, since the amount of solid solution Mo is small compared with this invention example (steel plate numbers A1, A11) of the same composition, local corrosion resistance is slightly inferior. However, compared with the comparative example, corrosion resistance is remarkably excellent.

Steel sheet No. A26 also satisfies the present invention as a chemical composition, but the total amount of solid solution Mo and solid solution W is slightly smaller than that of the present invention examples (steel plate numbers A6 and A13) of the same composition, and therefore, the local corrosion resistance is slightly inferior. However, compared with the comparative example, corrosion resistance is remarkably excellent.

From the local corrosion characteristics shown in Table 4, the overall corrosion characteristics shown in Table 5, and the amount of sludge formation, almost all of the Cu, Mo, and W, which are essential elements of the present invention, are contained in the composition of ordinary steel. Compared with the steel plate number B1 of the comparative example, the corrosion rate and the sludge formation rate of this invention steel are suppressed to about 1/4 or less, and it is clear that corrosion resistance is remarkably improved. Particularly, in regard to the local corrosion resistance shown in Table 4, in the present invention, the micro segregation portion has less micro segregation or micro segregation is reduced by diffusion heat treatment, and Mn is concentrated 1.2 times or more than the average Mn% of steel. As for the area ratio of 10% or less, local corrosion resistance improvement is aimed at further.

On the other hand, since steel plate numbers B1-B9 do not satisfy the requirements of the present invention, they are inferior in corrosion resistance to the present invention.

That is, the steel plate number B1 (steel sheet number 31) does not contain all of Cu and Mo and / or W which are essential for local corrosion resistance and sludge formation suppression, and as a result, solid solution Mo and W amount cannot necessarily be secured. , Local corrosion resistance, total corrosion resistance, sludge resistance, all significantly inferior to the present invention.

Although steel plate number B2 (steel plate number 32) contains Cu, since it does not contain Mo and W, both local corrosion resistance, total corrosion resistance, and sludge resistance are inferior to the example of this invention.

Although steel plate number B3 (steel plate number 33) contains Mo but does not contain Cu, the effect of this invention cannot be exhibited, and both local corrosion resistance, total corrosion resistance, and sludge resistance are inferior to the example of this invention.

Since steel plate number B4 (copper number 34) has an excessive amount of Cr, corrosion resistance is inferior to this invention. In particular, in corrosion conditions with high salt concentration (corrosion conditions ② in Table 4), the deterioration of local corrosion resistance is not preferable because of deterioration compared to ordinary steel.

Since steel plate number B5 (steel plate number 35) contains excess P, local corrosion resistance, total corrosion, and sludge resistance are inferior to the example of this invention. The amount of sludge produced tends to increase.

Since steel plate number B6 (steel plate number 36) contains excess S, both local corrosion resistance, total corrosion resistance, and sludge resistance are inferior to the example of this invention. The amount of sludge produced tends to increase.

Since steel plate number B7 (steel plate number 37) is less than the lower limit of the range of this invention, local corrosion resistance is inferior to the example of this invention. The amount of sludge produced tends to increase.

Since steel plate number B8 (steel plate number 38) contains excessively Al, local corrosion resistance is inferior to the example of this invention. The amount of sludge produced tends to increase. Toughness also drops.

Since steel plate number B9 (steel plate number 39) contains excessive Mo, local corrosion resistance is inferior to the example of this invention. The amount of sludge produced tends to increase. Moreover, since it is inferior to toughness and weldability, it is not preferable.

From the above examples, according to the present invention, against crude oil corrosion occurring in the constituent oil tank for transporting or storing crude oil, it shows excellent overall corrosion resistance and local corrosion resistance, and also suppresses the formation of corrosion products (sludge) including solid S. It is obvious that it can be done.

TABLE 1

[Continued Table 1]

TABLE 2

Note 1) In case of converter-continuous casting, the slabs include slabs intact and after-cast and rolled.

In the case of vacuum melt ingots, the thickness of the ingot is the thickness of the slabs.

Note 2) AC: air cooled, FC: low cooling

Note 3) Actual value in hot work test simulating actual history of rolling.

[Continued Table 2]

Note 1) In case of converter-continuous casting, the slabs include slabs intact and after-cast and rolled. In the case of vacuum melt ingots, the thickness of the ingot is the thickness of the slabs.

Note 2) AC: air cooled, FC: low cooling

Note 3) Actual value in hot work test simulating actual history of rolling.

[Continued Table 2]

Note 1) In case of converter-continuous casting, the slabs include slabs intact and after-cast and rolled. In the case of vacuum melt ingots, the thickness of the ingot is the thickness of the slabs.

Note 4) Air conditioners without accelerated cooling are not listed.

Note 5) Average cooling rate from accelerated cooling stop to 100 ° C.

Note 6) Failure to state conditions does not comply with

Note 7) Ac ₃ transformation point at elevated temperature at the time of collimation.

Note 8) Average cooling rate of 700 to 300 ° C

Note 9) All cooling is air cooling. It is not considered if there is no mention.

Note 10) The area ratio of the steel sheet in which the Mn concentration is 1.2 times or more the average Mn concentration when the 5 mm x 5 mm region is measured by the X-ray microanalyzer.

[Continued Table 2]

Note 1) In case of converter-continuous casting, the slabs include slabs intact and after-cast and rolled. In the case of vacuum melt ingots, the thickness of the ingot is the thickness of the slabs.

Note 4) Air conditioners without accelerated cooling are not listed.

Note 5) Average cooling rate from accelerated cooling stop to 100 ° C.

Note 6) Failure to state conditions does not comply with

Note 7) Ac ₃ transformation point at elevated temperature at the time of collimation.

Note 8) Average cooling rate of 700 to 300 ° C

Note 9) All cooling is air cooling. It is not considered if there is no mention.

Note 10) The area ratio of the steel sheet in which the Mn concentration is 1.2 times or more the average Mn concentration when the 5 mm x 5 mm region is measured by the X-ray microanalyzer.

TABLE 3

Note 1) Test pieces are taken from the plate thickness center in the direction perpendicular to the rolling direction.

Note 2) Conforms to JIS Z3103

TABLE 4

Note 1) Relative value at which the corrosion rate of Comparative Example B1 is 100

Corrosion Rate of Comparative Example B1

Corrosion condition ① 0.56mg / ㎠ / h

Corrosion Condition② 16.2mg / ㎠ / h

Note 2) Corrosion condition ①: pH0.5 (1 vol% HCl + 10mass% NaCl-30 ° C × 24h)

Note 3) Corrosion conditions ②: pH 0.2 (1 vol% HCl + 20 mass% NaCl-30 ° C x 24 h)

TABLE 5

Note 1) Relative value at which the corrosion rate (0.54 mm / y) of Comparative Example B1 is 100.

Note 2) Relative value where the mass (1260 mg / test specimen) of the corrosion product including the precipitated solid S of Comparative Example B1 was 100.

TABLE 6

According to the present invention, corrosion resistance to crude oil generated in oil tanks of crude oil tankers or constituent tanks for transporting or storing crude oil such as above-ground or underground crude oil tanks exhibits excellent overall corrosion resistance and local corrosion resistance and also contains solid S. Crude oil tanks for welded structures and crude oil tanks that can suppress the production of products (sludge) can be provided, contributing to long-term reliability improvement, safety improvement, and economical efficiency of steel structures and vessels. Therefore, the industrial effect of this invention is extremely large.

Claims (17)

In mass%, C: 0.001-0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, P: 0.03% or less, S: 0.007% or less, Cu: 0.01 to 1.5%, Al: 0.001-0.3%, N: 0.001-0.01%, A crude oil tanker steel further comprising one or two of Mo: 0.01 to 0.2% and W: 0.01 to 0.5%, with the balance being made of Fe and unavoidable impurities. The crude oil tanker steel according to claim 1, characterized in that by mass%, solid solution Mo + employment W ≧ 0.005%. The crude oil tanker steel according to claim 1 or 2, wherein the carbon equivalent (Ceq.) Represented by the formula (1) is 0.4% or less in mass%. Ceq. = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + W + V) / 5. ① The crude oil tanker steel according to any one of claims 1 to 3, wherein the mass% contains less than 0.1% Cr. The crude oil tanker steel according to any one of claims 1 to 4, further comprising one or two kinds of Ni: 0.1 to 3% and Co: 0.1 to 3%. The mass% according to any one of claims 1 to 5, Sb: 0.01 to 0.3%, Sn: 0.01 to 0.3%, Pb: 0.01 to 0.3%, As: 0.01 to 0.3%, Bi: Crude oil tanker steel further containing 1 type (s) or 2 or more types in 0.01 to 0.3%. The mass% according to any one of claims 1 to 6, Nb: 0.002-0.2%, V: 0.005-0.5%, Ti: 0.002-0.2%, Ta: 0.005-0.5%, Zr: 0.005-0.5%, B: Crude oil tanker steel further containing 1 type (s) or 2 or more types in 0.0002 to 0.005%. The mass% according to any one of claims 1 to 7, Mg: 0.0001% to 0.01%, Ca: 0.0005% to 0.01%, Y: 0.0001 to 0.1%, La: 0.005 to 0.1%, Ce: Crude oil tanker steel further containing 1 type (s) or 2 or more types in 0.005 to 0.1%. The crude oil tanker steel according to any one of claims 1 to 8, wherein an area ratio of the micro segregation portion in which Mn is concentrated 1.2 times or more than the average Mn% of the steel is 10% or less. It is a method of manufacturing the crude oil tank steel in any one of Claims 1-9. When carrying out accelerated cooling of the steel piece which consists of a component of any one of Claims 1-8 after hot rolling, an average cooling rate: 5-100 degreeC / s, acceleration cooling stop temperature: 600 degreeC-300 degreeC, acceleration Cooling rate to -100 degreeC after cooling stop: 0.1-4 degreeC / s, The manufacturing method of the crude oil tank steel characterized by the above-mentioned. A method for producing a crude oil tanker steel, wherein the steel produced by the method according to claim 10 is tempered or annealed at 500 ° C. or lower. It is a method of manufacturing the crude oil tank steel in any one of Claims 1-9. The heating temperature of the casting is: when the steel piece composed of the component according to any one of claims 1 to 8 is produced by normalizing after hot rolling, an average of Ac ₃ transformation point to 1000 ° C and 700 to 300 ° C. Cooling rate: 0.5-4 degreeC / s, The manufacturing method of the crude oil tank steel. After the annealing according to claim 12, an annealing or annealing is performed at 500 ° C or lower, characterized in that the method for producing crude oil tank steel. The heating piece according to any one of claims 10 to 13, before the hot rolling of the steel sheet comprising the component according to any one of claims 1 to 8, the heating temperature: 1200 to 1350 ° C, and the storage time: 2 to 100 A method for producing crude oil tank steel, which is subjected to diffusion heat treatment for a time. Crude oil tank, characterized in that part or all of the bottom plate, deck plate, side plate and aggregate are made of the crude oil tanker steel according to any one of claims 1 to 9. The method of the crude oil tank, wherein the hot rolled scale on the surface of the crude oil tank according to claim 15 is mechanically or chemically removed to expose the iron raw material. 17. The method of claim 16, wherein the hot rolled scale is mechanically or chemically removed, and then at least one layer having a thickness of 10 µm or more is formed.